This invention relates to the field of semiconductor light emitting devices, and particularly relates to current confinement within lasers.
The typical semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap, low refractive index layers. The low bandgap layer is termed the "active layer", and the bandgap and refractive index differences serve to confine both charge carriers and optical energy to the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. The cladding layers have opposite conductivity types and when current is passed through the structure, electrons and holes combine in the active layer to generate light.
Several types of surface emitting lasers have been developed. One such laser of special promise is termed a "vertical cavity surface emitting laser" (VCSEL). (See, for example, "Surface-emitting microlasers for photonic switching and inter-chip connections," Optical Engineering, 29, pp. 210-214, March 1990, for a description of this laser. For other examples, note U.S. Pat. No. 5,115,442, by Yong H. Lee et al., issued May 19, 1992, and entitled "Top-emitting surface emitting laser structures," which is hereby incorporated by reference, and U.S. Pat. No. 5,475,701, issued Dec. 12, 1995, by Mary K. Hibbs-Brenner, and entitled "Integrated laser power monitor," which is hereby incorporated by reference. Also, see "Top-surface-emitting GaAs four-quantum-well lasers emitting at 0.85 .mu.m," Electronics Letters, 26, pp. 710-711, May 24, 1990.) The laser described has an active region with bulk or one or more quantum well layers. The quantum well layers are interleaved with barrier layers. On opposite sides of the active region are mirror stacks which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength (in the medium) of interest thereby forming the mirrors for the laser cavity. There are opposite conductivity type regions on opposite sides of the active region, and the laser is turned on and off by varying the current through the active region. However, a technique for digitally turning the laser on and off, varying the intensity of the emitted radiation from a vertical cavity surface emitting laser by voltage, with fixed injected current, is desirable. Such control is available with a three terminal voltage-controlled VCSEL described in U.S. Pat. No. 5,056,098, by Philip J. Anthony et al., and issued Oct. 8, 1991, which is hereby incorporated by reference.
For several reasons, it is desirable to use surface emitting devices. For example, surface emitting devices can be fabricated in arrays with relative ease while edge emitting devices can not be as easily fabricated into arrays. An array of lasers can be fabricated by growing the desired layers on a substrate and then patterning the layers to form the array. Individual lasers may be separately connected with appropriate contacts. Such arrays are potentially useful in such diverse applications as, for example, image processing inter-chip communications, i.e., optical interconnects, and so forth. Second, typical edge-emitter lasers are turned on and off by varying the current flow through the device. This requires a relatively large change in the current through the device which is undesirable; the surface emitting laser, described below, requires lower drive current, and thus the change of current to switch the VCSEL need not be as large.
Top-surface-emitting AlGaAs-based VCSELs are producible in a manner analogous to semiconductor integrated circuits, and are amenable to low-cost high-volume manufacture and integration with existing electronics technology platforms. Moreover, VCSEL uniformity and reproducibility have been demonstrated using a standard, unmodified commercially available metal organic vapor phase epitaxy (MOVPE) chamber and molecular beam epitaxy (MBE) giving very high device yields.
In FIG. 1 is a diagram of a two terminal VCSEL 10. Formed on an n+ gallium arsenide (GaAs) substrate 14 is an n-contact 12. As indicated, substrate 14 is doped with impurities of a first type (i.e., n type). An n- mirror stack 16 is formed on substrate 14. Formed on stack 16 is a layers 18. Layers 18 has a bottom n-confinement or spacer layer 20 formed on stack 16, an active region 22 formed on layer 20 and a top n-confinement or spacer layer 24 formed on active region 22. A p- mirror stack 26 is formed on top confinement layer 24. As noted, sometimes confinement layers 20 and 24 may be referred to as spacers with active region 22 in between them. A p- metal layer 28 is formed on stack 26. The emission region may have a passivation layer 30. Isolation region 29 restricts the area of the current flow 27 through the active region. Region 29 can be formed by deep H+ ion implantation or by other known techniques.
Layers 18 may contain quantum wells disposed between mirror stacks 16 and 26. Stacks 16 and 26 are distributed Bragg reflector stacks. Quantum well active region 22 has alternating layers of aluminum gallium arsenide (AlGaAs) barrier layers and GaAs well layers. Stacks 16 and 26 have periodic layers of doped AlGaAs and aluminum arsenide (AlAs). The AlGaAs of stack 16 is doped with the same type of impurity as substrate 14 (e.g., n type), and the AlGaAs of stack 26 is doped with the other kind of impurity (i.e., p type).
Contact layers 12 and 28 are ohmic contacts that allow appropriate electrical biasing of laser diode 10. When the p-n junction of the laser is forward biased with a more positive voltage on contact 28 than on contact 12, current flows from contact 28 on through the stacks to contact 12, and active region 22 emits light 32 which passes through stack 26.
There may be a saturable absorber, such as layer 25, composed of GaAs, for example, to absorb light at wavelengths (.lambda.) less than 870 nm or composed of In.sub.x Ga.sub.1-x As quantum wells (such as 80 angstroms in thickness and wherein x may be 0.2 as an example) to absorb light at wavelengths less than one micron. Layer 25 can be of a .lambda./4 order in thickness but need not be such. Layer 25 can be situated anywhere in the stack of device 10. Instead, for instance, layer 31 may be the saturable absorber. Placement of the saturable absorber at a position in the cavity within layers 16 through 26 is influenced by the confinement factor. The saturable absorber may also be placed within spacer regions 20 or 24. For example, saturable absorber layer 25 may be a kth distance of k one-quarter wavelengths from active region 22.
A three terminal version of a vertical cavity surface emitting laser is shown in a sectional view in FIG. 2, in contrast to the two terminal VCSEL of FIG. 1. The frequency of the self pulsations of VCSEL 60 light 70 may be modulated or tuned with a drive power of varying amplitude between terminals 52 and 50. A saturable absorber may be situated anywhere between terminals 50 and 52. This inexpensive, low power device 60 has a significant frequency modulation bandwidth. The application of current across terminals 48 and 50 of VCSEL 60 can be constant, but tuned to give the right center self-pulsation frequency and/or light output. This configuration would result in minimal amplitude modulation of the VCSEL 60 light 70, as opposed to a two-terminal current-injected frequency-modulated VCSEL. Typically, the three terminal device 60 has a fixed constant current between terminals 48 and 50 resulting in a particular voltage-current (VI) (reverse or forward biased) being applied between those terminals.
As will be appreciated by those skilled in the art, some elements which are not essential to an understanding of the invention are either not depicted or described in detail. For example, only a single laser is illustrated in FIG. 2, although it will be readily noted that an array of lasers typically may be present. Shown are substrate 34, regions 36 and 46 having a first conductivity type, active region 38, regions 40 and 42 having a second conductivity type, with saturable absorption region 44 having either or neither conductivity type depending on design and operating conditions. Generally, the first conductivity is n type and the second is p type. Region 44 may comprise any number of bulk materials or one or more quantum wells, being normally absorbing at the lasing wavelength. Regions 36, 40, 42 and 46 comprise mirrors which are depicted as interference mirrors. Region 36 comprises a first mirror. Only several layers are shown for reasons of clarity. Appropriate regions of different conductivity types will be readily selected by those skilled in the art. Regions 40, 42, 44 and 46 form a second distributed mirror with a cavity Q and hence an oscillation frequency controllable via power applied to the saturable absorber through contacts 50 and 52. The active region typically comprises one or more quantum well regions which are interleaved with barrier layers, i.e., layers having a bandgap greater than the bandgap of the quantum well region. However, the use of bulk semiconductors instead is not precluded. There are first, second, and third electrical contacts 48, 50 and 52, to region 36, region 40 and layer 46, respectively. Contact 48 may be physically made to substrate 34 if the substrate is conducting and not semi-insulating. Isolation region, mode control or current confinement 54 restricts the area of the current flow through the active region to the area generally under region 46. Isolation region 54 can be formed by, e.g., deep ion implantation. Other forms of current and optical confinement may be utilized. The portions of regions 36 and 40 having first and second conductivity types, form means for injecting carriers into the active region. The first and second interference mirrors further comprise a plurality of interleaved first and second semiconductor layers with each layer having characteristics such that it is typically a quarter wavelength thick at the medium wavelength of interest thereby forming the respective interference mirror. The individual layers of the active region and the interference mirrors are not described with particularity as those skilled in the art know the structure of these elements.
Substrate 34 is conducting or semi-insulating GaAs, and regions 36, 40, 42 and 46 comprise alternating layers of AlAs and AlGaAs, as an example, with properties as described in the previous paragraph. The active region may comprise one or multiple GaAs (or, e.g., In.sub.x Ga.sub.1-x As) quantum wells interleaved with AlGaAs barrier layers. Saturable absorption (SA) region 44 is optically coupled to region 40, i.e., the absorption due to the SA is within the distributed mirror incorporating regions 40, 42, 44 and 46. Region 46 comprises interference mirror layers of, e.g., AlAs and AlGaAs, and has a first conductivity type. Those skilled in the art will readily select appropriate layer thicknesses and these parameters need not be described in detail. The use of other semiconductors is contemplated and appropriate choices will readily be made by those skilled in the art. For instance, other Group III-IV semiconductors may be used.
Conventional and well-known epitaxial growth techniques, such as molecular beam epitaxy or metallo-organic chemical vapor deposition, may be used to grow the layers described. After the layers have been grown, conventional patterning techniques are then used to form the individual lasers in the array. Electrical contacts to the individual lasers are also fabricated. Those skilled in the art will readily select appropriate patterning and contacting techniques.
The frequency of oscillation of the self-pulsing light emitted from the device can be varied by controlling the properties of the SA region within the VCSEL structure. An embodiment may use current or voltage alteration of bulk or quantum-well material such as the quantum-confined Stark effect in quantum wells. This effect is well known and understood by those skilled in the art; the effect is described in Chapter 4 entitled "Nonlinear optical properties of semiconductor quantum wells," by D. S. Chemla et al., in Optical Nonlinearities and Instabilities in Semiconductors, pp. 339-347, (Academic Press 1988). Basically, the absorption depends on the magnitude of the electric field in the quantum well.
A vertical cavity surface emitting laser needs relatively large reflectivities in both mirror stacks for lasing; typically, mirror stack reflectivities should be 99 percent or greater. The SA region functions as a bias-dependent absorber, by appropriately varying the bias, the laser pulsation can be frequency modulated at different rates. A small voltage or current change may be used to vary the absorption or carrier density of the SA and hence the frequency of the VCSEL self-pulsation. However, the magnitude of the current supplied through contacts 48 and 50 of device 60 of FIG. 2, may remain essentially constant as the laser is modulated. This simplifies the design of the power supply (not shown) for the array and minimizes any problems that might otherwise arise due to the varying heat generated in the vertical cavity laser array, due to the varying carrier density in the active region, and due to the resulting index changes in the optical cavity.
In FIG. 2, terminal 50 represents the top (usually the p type) contact and terminal 48 represents the bottom contact (usually the n type) contact. The bottom contact may be a common metalization on the bottom like contact 12 as shown in FIG. 1. Contact 52 represents a third connection which can be used to either reverse bias or forward bias the saturable absorber layer which is schematically illustrated by layer 44.
Light of the device may be emitted through either the substrate at one end or the top mirror at the other end. It will also be understood that the term, "vertical," is used to mean perpendicular to the major surfaces of the substrate. The means for injecting power can have first and second conductivity types on opposite sides of the active region, either along the axis formed by the first mirror, active region and second mirror, or along some other axis.